The present invention relates generally to 3-dimensional (3D) computerized tomography (CT) and, more specifically, to an improved cone beam CT system and method for efficiently acquiring projection data for image reconstruction of a region of interest using circular scans, wherein the detector area is optimally utilized for all source positions.
A system employing cone beam geometry has been developed for three-dimensional (3D) computed tomography (CT) imaging that comprises a cone beam x-ray source and a 2D area detector. An object to be imaged is scanned, preferably over a 360 degree angular range and along its entire length, by any one of various methods wherein the position of the area detector is fixed relative to the source, and relative rotational and translational movement between the source and object provides the scanning (irradiation of the object by radiation energy). The cone beam approach for 3D CT has the potential to achieve 3D imaging in both medical and industrial applications with improved speed, as well as improved dose utilization when compared with conventional 3D CT apparatus (i.e., a stack of slices approach obtained using parallel or fan beam x-rays).
As a result of the relative movement of the cone beam source to a plurality of source positions (i.e., xe2x80x9cviewsxe2x80x9d) along the scan path, the detector acquires a corresponding plurality of sequential sets of cone beam projection data (also referred to herein as cone beam data or projection data), each set of cone beam data being representative of x-ray attenuation caused by the object at a respective one of the source positions. The cone beam projection data is then processed to reconstruct a 3D image of the object using image reconstruction methods known in the art.
Various methods have been developed for 3D image reconstruction for cone beam x-ray imaging systems. For example, a back projection cone beam image reconstruction technique is described in U.S. Pat. No. 5,257,183, which issued on Oct. 26, 1993 to Kwok C. Tam, entitled xe2x80x9cMethod and Apparatus for Converting Cone Beam X-Ray Projection Data To Planar Integral and Reconstructing a Three-Dimensional Computerized Tomography (CT) Image of an Objectxe2x80x9d, which is incorporated herein by reference. This patent discloses a method and apparatus for converting cone beam data to values representing planar integrals on any arbitrary set of planes in Radon space for 3D image reconstruction through an inverse Radon transformation. Back projections can be mathematically accomplished for a cone beam source by inverse Radon transforming suitable planar integrals. The planar integrals are computed from detector integrals which utilize the measured cone beam projection data, i.e., the detected attenuated intensity representative of the density distributions of the object. The use of a cone beam source expedites data acquisition. A direct Radon inversion of three dimensional cone beam data from a cone beam source is not possible. Thus, before an inverse Radon transform can be undertaken in a three dimensional cone beam data implementation, the cone beam detector integrals must be reconfigured into planar integrals suitable for inverse Radon transformation. U.S. Pat. No. 5,257,183 discloses a method for image reconstruction by calculating Radon derivative data from the acquired cone beam data. The Radon derivative data is typically determined by calculating line integrals for a plurality of line segments drawn in the acquired cone beam data. Radon space driven conversion of the derivative data is used to develop an exact image reconstruction of a region-of-interest (ROI) in the object.
FIG. 1 illustrates a typical circular scanning geometry for three dimensional CT scanning employing cone beam geometry. An object 10 to be imaged is positioned within a field of view between a cone beam point source 11 and a two dimensional detector array 12 that acquires cone beam projection data. An axis of rotation 13 passes through the field of view and the object 10. A midplane 14 is defined as the plane that (i) is normal to the axis of rotation 13 and (ii) contains the cone beam point source 11. In the exemplary embodiments described herein, the axis of rotation 26 is the v axis, having its origin (0,0,0) at its intersection with the midplane. The coordinate system is fixed relative to the source 11 and detector 12. When scanning the object 10 at a plurality of angular positions, the source 11 moves relative to the object 10 and the field of view rotates along a circular scanning trajectory 15 lying in the midplane 14, while the detector 12 remains fixed with respect to the source 11 (or alternatively the object 10 can be rotated while the source 11 and detector 12 remain stationary). Data is acquired at a plurality of source positions during the scan. Data collected at the detector 12 represent line integrals through the object 10. The approach to reconstruction then embodies calculating planar integrals on a set of planes from various line integrals through the object, then performing an inverse Radon transform on the planar integrals to reconstruct a three dimensional image of the object. It is known that data collected in such a single circular scan is incomplete and artifacts may be introduced into the reconstructed image.
One image reconstruction method that uses two circular scans and a Radon transform approach to three dimensional CT imaging is disclosed, for example, in U.S. Pat. No. 5,383,119, which issued on Jan. 17, 1995 to Kwok Tam, entitled xe2x80x9cMethod and Apparatus For Acquiring Complete Radon Data for Exactly Reconstructing a Three Dimensional Computerized Tomography Image Of a Portion of an Object Irradiated By a Cone Beam Sourcexe2x80x9d, which is incorporated herein by reference. This patent discloses a method for imaging a region of interest (ROI) in a long object by scanning the ROI in 2 circular scan paths and a line scan that connects the two circular scans. The method enables exact 3D reconstruction of the image of a portion of interest of an object in the field of view of a cone beam source by selectively disregarding unwanted Radon data which may corrupt the imaging process and selectively recovering Radon data that would otherwise be lost. The method enables acquisition of a complete Radon data set through proper choice of scanning configuration and selective partitioning and manipulation of the acquired data.
More specifically, referring to FIG. 2, a ROI xcexa9o within a long object 20 (e.g., human body) is reconstructed by cone beam data collected by (i) an upper scan trajectory 21 and lower scan trajectory 22, which are taken to be spaced circular trajectories that are normal to the axis of rotation v, and (ii) a connecting line scan. A circular scan is performed for both the upper and lower bounds of the ROI and the data from the two circular scans are combined in such a manner as to appropriately reconstruct the ROI. FIG. 3 depicts the method for combining the cone beam data from two circular scan paths for reconstructing the ROI in a long object, wherein cone beam source positions A and B correspond to the top and bottom scans, respectively. In general, processing the cone beam data for an exact image reconstruction involves filtering, either implicitly or explicitly, all line segments on the detector (i.e., the integration line segments). In the data combination process, at each cone beam view, the line segments being filtered are restricted to only the cone beam data bound by the angular ranges as depicted in FIG. 3. In this manner, the totality of the cone beam data from all the contributing source positions covers every plane of integration intersecting the ROI in its entirety without any overlap.
More specifically, an image reconstruction method described in the above-incorporated U.S. Pat. No. 5,383,119 comprises the steps of performing a circular scan using a cone beam source along a circular path enclosing a ROI at the upper and lower extent of the ROI and joining the upper and lower circular scan paths by a connecting scan path. The upper, lower, and connecting scan paths collectively define a complete scan path for the ROI. Then, each plane of integration corresponding to a data point in Radon space is selectively partitioned based on the intersection of the plane with the ROI. Then, based on the partitioning, the cone beam data is selectively manipulated to discard data that is not directly attributable to the ROI and to recover otherwise missing data directly attributable to the ROI.
The process of selectively portioning each plane of integration comprises categorizing each plane of integration based on the manner in which the plane intersects the ROI. More specifically, to assess the adequacy of the filling of Radon space with Radon data, each integration plane that contributes to points in Radon space is categorized as follows: (i) a plane that does not intersect the ROI; (ii) a plane that intersects the ROI only; (iii) a plane that intersects the ROI and also either the region above the ROI or the region below the ROI, but not both; and (iv) a plane that intersects the ROI and also both the region above and the region below the ROI.
The process of selective manipulation generally comprises selectively disregarding cone beam data contributed from regions beyond the ROI in the long object to eliminate otherwise image corrupting data and selectively combining cone beam data obtained from the upper and lower scans to recover data otherwise missing due to corruption by regions beyond the ROI. In particular, for case (i), the planar integral will be zero so the corresponding cone beam data is discarded. For case (ii), the planar integral is computed from the cone beam data based on the methods described in the above-incorporated U.S. Pat. No. 5,257,183. For case (iii), the planar integral is computed from the top scan data or the corresponding bottom scan data as follows: the unwanted contribution of that portion of the object beyond the ends of the ROI to the computation of the planar integrals is eliminated by discarding all the cone beam data whose paths traverse the region beyond the ends of the ROI. For case (iv), the data is selectively manipulated by selectively combining cone beam data from top and bottom level scans for each plane intersecting both the top and bottom levels of the ROI, as diagrammatically shown in FIG. 3. Finally, missing Radon data is then filled in by any scan connecting the upper and lower circular scans. Further details of the above method for acquiring complete Radon data for exact image reconstruction of a 3D image of the ROI can be found in the above-incorporated U.S. Pat. No. 5,383,119.
One known method for restricting the cone beam data to the appropriate angular range is accomplished by a xe2x80x9cmaskingxe2x80x9d process, which facilitates efficient 3D CT imaging when only the ROI in the object is to be imaged, as is normally the case. During a scan, the scanning trajectory is sampled at a plurality of source positions where cone beam energy is emitted toward the ROI. After passing through the ROI the residual energy at each of the source positions is acquired on the area detector as a given one of a plurality of sets of cone beam data. Each set of the cone beam data is then xe2x80x9cmaskedxe2x80x9d so as to remove a portion of the cone beam data that is outside a given sub-section of a projection of the ROI in the object and to retain cone beam projection data that is within the given sub-section. The shape of each mask for a given set of cone beam data will vary based on the scan orbit. The cone beam data that is retained (via the masking process) is then processed so as to develop reconstruction data. An exact image of the ROI is developed by combining the reconstruction data from the various source positions which intersect a common integration plane. As such, the masks are commonly referred to as xe2x80x9cdata-combinationxe2x80x9d masks.
Typically, and in a preferred embodiment of the present invention, projection data for image reconstruction is acquired by applying a xe2x80x9cmaskxe2x80x9d to each set of the projection data so that data inside the boundaries of each mask form a corresponding plurality of masked 2D data sets.
FIG. 4 is a diagram that illustrates masks used in the exemplary embodiments of FIGS. 2 and 3, wherein circular scans are taken along the upper and lower boundaries of the ROI. FIG. 4a illustrates the upper circular scan trajectory 21 and lower circular scan trajectory 22 which are separated by a distance of 2H (line scan) and each trajectory having a diameter of 2R. FIG. 4b illustrates a mask M which is applied to the cone beam data on detector D for the upper circular scan 21. As shown, the mask M boundaries comprise a top curve and a bottom curve, which are formed by projecting the upper circular scan path from a current source position. More specifically, for a flat detector D located at the rotation axis such that the line connecting the source to the detector origin is normal to the detector plane, the equation of the mask M for a given source position on the top circular scan path is defined by the following:
Top Boundary: v=0
Bottom Boundary   v  =      -          H      ⁡              (                  1          +                                    u              2                                      R              2                                      )            
where u and v are the Cartesian coordinate axes of the detector with the v axis coinciding with the rotation axis and R is the radius of the circles. FIG. 4c illustrates a mask M for the bottom circular scan 22, which is just the inverse of the mask for the upper scan.
As discussed above, in medical x-ray scanners, the source and the area detector are always fixed relative to each other, with the source projecting onto the center of the detector. In such a framework, when the data masks illustrated in FIG. 4 are used to filter the cone beam data, only one half of the detector area is essentially utilized in capturing data at each of the circular scans, while the other half of the detector is not utilized at all. For instance, as shown in FIG. 4b, the cone beam data in the upper half of the detector, i.e., v greater than 0, is filtered out via the mask and consequently, that portion of the detector is essentially not used for capturing data during the circular scan.
Accordingly, an improved image reconstruction method that would enable efficient utilization of the detector area, is highly desirable.
The present invention is directed to a method for cone beam region of interest imaging of long objects with an area detector employing a series of circular scans, wherein the detector area is essentially fully utilized for all source positions.
In one aspect of the invention, a method for imaging a region of interest (ROI) of an object uses cone beam computed tomography, wherein a cone beam source is fixed with respect to a two dimensional detector. The method comprises performing a first and second circular scan along a first and second extent, respectively, of a ROI within an object being imaged. The cone beam source and detector are fixed such that during the first and second circular scans, one half of the detector area is used to acquire cone beam projection data corresponding to the ROI and a second half of the detector area is used to acquire cone beam projection data corresponding to a first and a second extended portion of the object, which extend beyond the first and second extents, respectively, of the ROT. A line scan is taken along the object connecting the first and second circular scans. The ROI is reconstructed by selectively combining the cone beam projection data corresponding to the ROI acquired from the first and second circular scans, and the first and second extended portions of the object are reconstructed using the cone beam projection data corresponding to the extended portions acquired from the first and second circular scans and the cone beam projection data acquired from the line scan. An extended ROI is reconstructed by combining the reconstructed ROI and reconstructed first and second extended portions
These and other objects, features and advantages of the present invention will be described or become apparent from the following detailed description of preferred embodiments, which is to be read in connection with the accompanying drawings.